In the course of performing various downhole well servicing operations, it is necessary to raise and lower a service tool within the well to set, position or release downhole equipment. The sucess of such operations is dependent upon the ability to reciprocate the tool a predetermined distance relative to the downhole equipment, and moreover, to reliably detect not only the fact of a desired displacement, but also that the displacement has occurred relative to a specific item of downhole equipment.
For example, such a requirement is essential for the successful installation of a gravel pack. In one class of such equipment, a service seal unit carried by a work string is reciprocated relative to certain flow ports and sealing points within a packer bore to route fluid along various passages. The service seal unit carries vertical and lateral circualtion passages which, when aligned with ports formed in a packing unit, permits service fluids such as acids, polymers, cements, sand or gravel-laden liquids to be injected into a formation through the bore of the work string and into the outer annulus between the sand screen and the perforated well casing, thereby avoiding plugging or otherwise damaging the sand screen.
In another position of the service seal unit, the annulus below the packer is sealed and the lateral flow passages of the service seal unit are positioned for discharge directly into the annulus between the work string and the well casing, thereby permitting reverse flow of clean-out fluids through the annulus between the work string and the well casing and upwardly through the bore of the work string.
The position of lateral flow passages and seals carried by the service unit must be closely controlled to insure that such service operations are performed correctly.
The foregoing procedures become more difficult in performing service operations on an off-shore well. Service operations in water depths exceeding a few hundred feet are generally performed from a floating, semi-submersible platform, or from a ship, which are supported by buoyancy and not from the sea bottom. In such operations, a marine riser connects well head equipment to a surface facility to provide a stable conduit through which the production or work string is extended and production fluids are conveyed to the service vessel.
The marine riser and work string cannot withstand compression loading and therefore must be supported under tension at the water surface to prevent collapse. This is easily accomplished when the surface facility is a production platform which is fixed to the ocean floor, but a more difficult problem is presented when the water depth is so great that the surface facility must be floating and thus subjected to the effects of wave-induced heave and swell forces.